The invention relates to an electric lamp comprising a lamp vessel which is closed at one end with a tube which extends into the lamp vessel and is sealed to current-supply conductors in a vacuum-tight manner at an end located inside the lamp vessel by means of a pinch. The current supply conductors are connected to respective contacts at the bottom of the lamp cap, and include a fuse wire which extends through an electrically insulating mass arranged between the relevant contact and the proximity of the pinch. Such a lamp is known from British Pat. No. 1060297 (General Electric Company, Mar. 1, 1967).
It is of great importance that electric lamps are provided with a fuse to interrupt the current circuit when an excessively high current flows through the lamp. However, when the fuse melts, a discharge arc can be produced which can flash over to the other current-supply conductor or to the sheath of the lamp cap. Such a discharge arc can weld the lamp cap to the lamp holder, blow the fuse of the equipment to which the lamp is connected, or cause the lamp to explode. Therefore, numerous proposals have been made to avoid these phenomena. In many cases, these proposals require additional parts to be used, as a result of which the manufacture of the lamp becomes more expensive.
In the lamp according to the aforementioned British patent, the tube is filled with quartz sand or with glass beads, the lamp cap is fixed on the lamp vessel and sealed with cement, and the lamp cap is filled for the remaining part with foamed synthetic material. As a result, the fuse is entirely incorporated in electrically insulating material and a discharge arc cannot be produced.
The foamed material in this lamp has a double function: it holds the grains of sand or glass in place and it insulates the fuse from the other current conductor and from the lamp cap (except for the relevant contact at the lamp cap).
It has been found that in practice this construction has great disadvantages. The synthetic material, from which the foam is formed upon heating, has to be pressed to form rings which are then fixed in the lamp cap. During the process of assembling the lamp vessel and the lamp cap, the lamp vessel should be arranged with its neck directed upwards in order to prevent the grains of glass or sand from flowing away, which implies that the lamp cap should be arranged with its opening downwards. The price of the lamps is increased by the steps of pressing the rings and fixing them. Moreover, this construction has the great disadvantage that while moving between the production machine for lamp caps and the assembling machine for lamps, the rings of foamed synthetic material can drop out of the lamp cap. As a result, the assembling machine should be provided with means for checking the presence of the rings in order to avoid producing some lamps in which the fuse wire is not satisfactorily enclosed by insulating material.